Ruminations on vocoders from Mark
Tom:
The vocoders in cellular are quite different from those in the PSTN, so a big difference is to be expected. Voice quality was never the objective in going to digital. Capacity was the objective. Also, the error correction in digital cellular is speed sensitive, due to the different types of radio frequency fading that predominates at different speeds. Vocoders used in digital cellular (or PCS) are very different in their operation from the A/D conversion used in CDs. It is a predictive process, and intended to maximize the bandwidth efficiency rather than providing superb voice quality. . ."